1. Field of the Invention
The present invention relates to a communication system, a method of controlling the same, and a storage medium.
2. Description of the Related Art
A control system for industrial equipment or the like generally includes an arithmetic processing unit (CPU or the like) and a plurality of sensors and actuators. A sensor acquires external information about a current value, and outputs it to the arithmetic processing unit. The arithmetic processing unit calculates the deviation from a target value based on the external information, and outputs a correction value thereof to the actuator. The actuator is driven based on the correction value. The control system achieves a desired operation by periodically repeating the above-described feedback control. One of the control systems is a robot system represented by an industrial robot.
Japanese Patent Laid-Open No. 09-029671 discloses a robot system which combines and connects mechanically and electrically independent joint modules, and transmits/receives information concerning operation control using communication units included in the joint modules, thereby controlling the overall operation. Note that as shown in FIG. 2 of Japanese Patent Laid-Open No. 09-029671, each joint module includes a communication unit and constitutes a network together with other joint modules. With this structure, a flexible robot system is constituted, which can do recombination in accordance with a change in operation contents even when a number of joint modules are combined.
Japanese Patent Laid-Open No. 2009-148845 discloses a robot system that recognizes operation contents, selects an appropriate end effector in accordance with the operation contents, and autonomously exchanges the end effector.
The period at which feedback control is performed is called a sampling period. Generally, when the sampling period is shortened, a control system capable of performing a fast and precise operation can be constituted. However, shortening the sampling period is restricted by several factors. One of the factors is a dead time. The dead time is a time delay that occurs in the control system. More specifically, it is a time delay necessary until a sensor acquires external information, and an actuator is driven based on a correction value calculated from the external information. The dead time needs to be sufficiently short as compared to the sampling period. If the dead time is too long relative to the sampling period to neglect, the feedback control is unstable. That is, to constitute a control system for implementing a fast and precise operation, the dead time needs to be made as short as possible. The dead time is represented by the sum of the transmission time necessary for information transmission and the arithmetic time necessary for the arithmetic processing unit to calculate the correction value from the external information.
The most simple arrangement for minimizing the dead time from the viewpoint of signal transmission is an arrangement that connects the arithmetic processing unit and all the sensors and actuators by wires in a one-to-one correspondence. This arrangement allows to greatly shorten the transmission time, which is, shorten the dead time. In this arrangement, however, cables are required as many as the sensors and actuators. The number of cables is enormous in a control system including a number of sensors and actuators. This poses problems such as an increase in the cable cost, complex cable leading, and the influence of the weight and vibration of the cables on control. To prevent this, networking the wires between the arithmetic processing unit and the sensors to save the wires has widely been addressed.
Japanese Patent Laid-Open No. 2006-94302 discloses a control system that line-connects a parent station incorporating an arithmetic processing unit and a child station to which a sensor and an actuator are connected. To form a network and shorten the dead time, the time (to be referred to as a “data relay time” hereinafter) necessary for each terminal station to relay data needs to be as short as possible.
Terminal stations connected to a network use an independent synchronization method and a slave synchronization method as synchronization methods. In the independent synchronization method, a station transmits data to a lower station using a clock oscillated by a locally provided clock oscillation unit, as shown in FIG. 10 of Japanese Patent Laid-Open No. 11-252144. For this reason, in the independent synchronization method, a rate match FIFO needs to be provided to absorb the transmission rate difference generated by the frequency deviation between the local clock and a reproduced clock reproduced by a clock recovery unit. On the other hand, in the slave synchronization method, a station transmits data to a lower station using a reproduced clock reproduced by a clock recovery unit, as shown in FIG. 6 of Japanese Patent Laid-Open No. 2003-209556. For this reason, in the slave synchronization method, no rate match FIFO needs to be provided, unlike the independent synchronization method.
When the two methods are compared from the viewpoint of shortening the dead time, it is preferable to employ not the independent synchronization method but the slave synchronization method. This is because the slave synchronization method, which need not arrange any rate match FIFO, can make the delay in data relay shorter by the time required for passing through the rate match FIFO, as compared to the independent synchronization method.
In the slave synchronization method, however, the total number of child stations connectable to the network is restricted, unlike the independent synchronization method. This is because in the slave synchronization method that uses a reproduced clock as the transmission clock, a jitter amount is added every time data passes through a child station, and as a result, a data error occurs through terminal stations of a predetermined number or more of stages. Each of Japanese Patent Laid-Open Nos. 62-72250 and 2008-145902 discloses a technique of increasing the total number of connectable child stations. Japanese Patent Laid-Open No. 62-72250 discloses a communication system in which a bypass is provided to transmit a clock between child stations that are not adjacent, and a clock generated in the child station closer to the parent station viewed from the data transmission direction is used the transmission clock of the child station far away from the parent station. Japanese Patent Laid-Open No. 2008-145902 discloses a communication system in which a clock transmission path for transmitting a clock reproduced in the child station closest to the parent station is provided, and each child station uses, as the transmission clock, one of a clock reproduced from a communication signal and the clock from the clock transmission path. Using these techniques makes it possible to suppress the jitter amount added every time data passes through a child station, that is, increase the total number of child stations connectable to the network, as compared to the simple slave synchronization method.
In Japanese Patent Laid-Open Nos. 62-72250 and 2008-145902, however, to increase the total number of child stations connectable to the network, a considerable number of cables need to be provided in accordance with the increase in the number of child stations in addition to those for data transmission. This poses problems such as an increase in the cable cost, complex cable leading, and the influence of the weight and vibration of the cables on control. From the opposite viewpoint, the smaller the number of cables other than those for data transmission is, the more the total number of child stations connectable to the network is restricted. That is, the total number of child stations that are connectable to the network and the wire saving of cables have a tradeoff relationship.